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Northwest Geology Field Trips, by Dave Tucker, is licensed under a Creative Commons Attribution-Noncommercial- Share Alike 3.0 United States License. You can use what you find here, repost it with attribution to the author, "remix" it for your own purposes, but may not use it with the intent of making money off of it.

EDUCATORS: Please feel free to use anything you find here that is useful to your mission educating people about Earth science. E-mail me if it would help to have a larger or higher-resolution version of any of the images. tuckerd at geol dot wwu dot edu

Aerial photo reveals the syncline-anticline pair at the north end of Chuckanut Mountain. Raptor Ridge is shown right-center. Click to enlarge any figure.

Here is a fine winter geology hike in the Chuckanut hills south of Bellingham. The hike is in the lowland forest the entire way, but has a nice variety of geologic features interspersed amongst the greenery: dipping strata, erosion, erratics (this website can’t seem to get away from the dang things), grooves in sandstone, and a great view out over the hills from a bare rocky ledge at Raptor Ridge.

These block models mimic the expression of the Chuckanut Mountain folds. NASA image.

Getting there: This geology hike begins at the Arroyo Park trailhead on Old Samish Road. The trailhead is 0.1 mile east of the junction of Old Samish and Chuckanut Drive (Hwy 11) south of Fairhaven. You can also start at the North Chuckanut Mountain Trailhead around the corner on Chuckanut Drive, but you will miss the narrow and pretty canyon of Chuckanut Creek. The moderately steep hike is 7.6 miles (12 km) round trip. It is a good workout to take the steeper 4.6 mile round trip hike from Pine & Cedar Lakes Trailhead on Lake Samish Road, but the geology is minimal on that route. Trails are well-signed at junctions. Here’s a map of trails in the Chuckanut system. Whatcom Transit route 105 gives access to the Interurban Trail on Old Fairhaven Parkway, 1 mile north of the Arroyo trailhead- you could do a bus/bike/hike from anywhere in town.

Weathered sandstone. Slug for scale (it was a big one).

The field trip: The trail follows the bank of Chuckanut Creek, eroded into the Chuckanut Formation. A few exposures of sandstone rise out of the stream, which can be pretty boisterous after heavy rains. See a YouTube video of December 2010 high water here (off-site). Cross the footbridge, and head up hill to the junction with the Hemlock Trail, which climbs up a steepish hillside. This is the dip slope on the northern limb of the Chuckanut anticline. On the way, you’ll pass the hulking Arroyo Park erratic, which I wrote about here (scroll down on the webpage). Take the branch trail that goes along the low side of the erratic, and you may spot a couple of ankle-high sandstone exposures in the trail tread that reveal the northerly dip of the Chuckanut, and which define the anticline around here. In a bit, reach the old road right of way and the trail becomes a wide thoroughfare. Pass the turn off to Lost Lake and walk along a swamp captured within eroded bedding planes of the Chuckanut. As you hike up the old road, watch for exposures of deeply weathered crumbly sandstone- hard to imagine it is actually rock in places. Once past the Huckleberry Point trail, there are more trailside exposures, but they aren’t exactly postcard or panoramio quality!

The 50 foot falls north of Hemlock Trail.

At 2.3 miles, stay left on the Hemlock Trail at the junction with the Salal Trail. The geology gets a little more interesting from here on, with a few Chuckanut outcrops. In the wet season (October to October) watch for a small creek; there is a 50 foot waterfall veiling a steep rock face in the bushes a few feet off to the left (if you don’t cross water, there ain’t no waterfall). The trail narrows to a ‘single track’. Watch for an 8-foot-tall

This green-draped gneiss erratic is close to the Raptor Ridge junction

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moss-enveloped trail-side erratic just after a right-handed switchback. You may have to look pretty hard to find a window through the green stuff, but you’ll be rewarded with the discovery of the boulder’s surface, which reveals gneiss. The rock is from somewhere to the north in BC, but I can say no more than that. After 3.3 miles of hiking, take the Raptor Ridge Trail to the right. Now the route gets more interesting, geologically, as the trail climbs steeply past high sandstone cliffs. Watch for places where a thin mossy veneer and weathering accentuate rippled beds in these alluvial sandstones. Note where an 18” tree has fallen over, revealing that it was only barely rooted into a thin rock fracture on the pitched slab beneath. One of the cool parts of this geohike is the gnarled Tolkienesque tree roots growing over and around rocks along this part of the trail.

ripple beds in Chuckanut sandstone near the top of Raptor Ridge.

The fallen tree's roots could only penetrate the narrow crack on the slab.

Tree roots uber alles!

Less than a half mile (800 m) from the Hemlock junction, pop out of the woods onto smooth slabs and expansive views at the top of a big cliff, ‘Raptor Ridge’, so named because birds of prey may be hurtling past, or casually cruising on updrafts. Enjoy the scenery, but don’t neglect the rock at your feet.

Looking south from Raptor Ridge. Glacial striae cut deeply into the soft sandstone.

There are two sets of grooves worn into the rock here, from two very different processes. The more subtle larger scale set of wider grooves were ground into the soft(ish) sandstone by the glacial rasp- ‘glacial striations’. The ice itself didn’t do this work, but rather the grit and rocks in the glacier’s base. These are oriented parallel to the vanished glacier’s direction of flow, north to south. Now, puzzle over the narrower deep branching grooves. What the heck? These are randomly oriented and terminating, inconsistent with glacial striae. I don’t know for certain, but the consensus among my geologist friends and one biologist is that these were made by now-gone tree roots, secreting weak tannic acid that dissolved the cement between the sand grains. They are post-glacial, since they are found on this glaciated surface. You’ll find grooves like this elsewhere in the Chucks, but the combination with the nice view and the great hike makes these the best local examples of the phenomenon that I know of. Imagine that, mere biology trumping geology. Hmmmmph.

The deep, narrow grooves at Raptor Ridge.

Back at the Hemlock Trail, you can turn right rather than returning left to continue along an old logging railroad grade to visit Pine and Cedar Lakes. You’ll join the steep trail up from Lake Samish Road 0.6 mile (1 km) beyond the Raptor Ridge junction (see the map link above). There is another high rock knob to the west of Cedar Lake reached by a quite short trail, not shown on the map, but pretty obvious. It takes off to the left from the railroad grade before reaching the trail to the lake itself. Traverse over the knob, which has different and more limited vistas from Raptor Ridge, and end up at the west shore of Cedar Lake.

The Hemlock Trail is a wonderful XC ski route in snowy winters. If you enjoy that, check out my other blog, Chuckanut Cross Country Skiing. The banner photo you’ll find there is on the Hemlock Trail a few years back.

Keith Kemplin skis the Hemlock Trail. Eat yer heart out!

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4 Responses

Thanks Tucker. Very interesting. We have been there many times and we have often had a snack or lunch there. We had noticed the glacial striations and the other “crack” but never gave much thought to how the other “crack” got there.

[…] For more information on the hike to Raptor Ridge, visit the Whatcom County Parks website, here http://www.co.whatcom.wa.us/parks/chuckanut/raptor-ridge.jsp; and for trip reports, go to the Washington Trails Association website, here – http://www.wta.org/go-hiking/trip-reports/trip_report.2012-04-29.1544498673. And for those of us who enjoy learning about the rocks along our trails, excellent information about the geology along the Raptor Ridge hike is provided here – https://nwgeology.wordpress.com/the-fieldtrips/the-chuckanut-formation/raptor-ridge-geology-hike-chuc…. […]

I already have the trip written up. There is some nice sedimentary structure detail. I have agreed with the trail builders and State Parks that I will publish it once the trail is formally opened in the Spring. In fact, I’m going to lead a geology field trip on the Rock Trail as a benefit for its maintainence. To be announced.
Dave